but then, you'd wind up with an expensive repair bill for an old power supply. Who knows what might break or burn out next?

Transformer still in it? Rectifiers still good? If so, you might just consider using some modern regulator IC's with some fresh electrolytics, using your existing meters, pots and enclosure. Of course, you'll need to test everything that remains to ensure it's not broken/burned out.

What was the output voltage and current rating of the power supply originally? That, along with how much time, effort and money you want to invest, will be a baseline for your re-engineered supply's capacity. There are single-chip solutions for outputs from 1.2-37v from 100mA to 7.5A; some of those chips can be made to jump through hoops as well. You can also "up the ante" by using power transistors (paralleled if need be) to output however much power your transformer can output.

I have just got my hands on a partially working Maplin PSU XG95D.
It is based on a LM723, which is a common power supply controller.
In addition a dual op-ampl LM324 is used as part of the current sense circuit and to switch in a higher voltage tap on the transformer.
I am trying to get a good circuit diagram. Did you get one?